Varian Associates, Lexington, MA

A novel event was part of the 30th
Anniversary celebration of the American Vacuum Society in 1983: On October 22,
1983, Boston’s Museum of Science and the New England chapter of the AVS
recreated von Guericke’s famous Magdeburg Hemispheres experiment. This activity
was organized in association with an exhibit of historical vacuum apparatus that
was first displayed at Boston’s Museum of Science before being moved and
displayed at the 30th AVS Symposium in Boston. A video tape of the
reenactment was shown repeatedly on a television monitor at the exhibit.

Six horses, three on each side,
pulled on evacuated steel hemispheres. Briefly, the first three attempts to
separate the hemispheres were not successful, but on the fourth try, the horses
were able to separate the two halves of the chamber.

Most comments overheard at the exhibit from
people watching the proceedings on the screen were that the separation was due
to a lateral shift of one side relative to the other. Specifically, this
explanation is incorrect because lateral restraints were incorporated internally
into one of the chambers to inhibit shifting due to a possible uneven transient
load during an initial jerk. The explanation for separation lies in the fact
that an impulse load developed by the horses was actually great enough to
overcome the force of the atmospheric pressure.

Even though the demonstration in Boston was
done in the spirit of fun, a number of technical problems had to be considered
during the design of the modern version of the hemispheres. They may be of some
interest to the readers.

The secret and the drama of the
demonstration lies in the question of coordination of the effort of the two
teams of horses as well as the simultaneous application of the pulling force by
each horse. If von Guericke’s experiment were to be scaled down to only one
horse (with a smaller chamber) and if one side of the evacuated chamber were
attached to a tree, the horse would have separated the chambers each time. von
Guericke may have been a better showman than a scientist, but, to be fair, it
must be noted that in this time Newton’s laws were unknown; force and momentum
were usually confused and energy considerations in impulse load calculations
were not appreciated.

As designers of the modern hemispheres, we
were so acutely aware of the possibility of embarrassment despite the use of
only three horses on each side that safety chains were provided to prevent
injury to the horses or to their handlers in case the half-chambers flew through
the air after separation.

The following considerations may be
illustrative. von Guericke usually used eight horses on each side and
hemispheres of 20 in. in diameter. The shape of the chambers is, of course,
immaterial. Only the projected force acting on the area enclosed by the sealing
circle should be considered. This gives an atmospheric pressure force of 4600
lb and the challenge of only 575 lb per horse. It would seem that even Don
Quixote’s Rosinante should have been able to produce that much force. As a rule
of thumb, a horse can produce a pulling force (standing on soft, grassy ground)
roughly equivalent to its own weight. 1 von Guericke undoubtedly was
clever enough to use light-weight carriage horses. At the Boston demonstration,
two teams of three horses each were used, each horse weighing over 1600 lb.

1
The Draft Horse primer, Maurice Telleen (Rodale Press, 1977)

There was another important distinction
between von Guericke’s and our demonstrations. The more horses are used, the
more difficult it is to have them pull in unison and, more importantly, to have
the two opposing teams apply a peak force simultaneously. The horses used in
Boston (Eastern Draft Horse Association) are trained for and accustomed to
participation in pulling contests. They usually produce a great exertion for
about 10 s pulling continuously a loaded sled approximately 30 ft. They seem to
be trained to move initially with sudden acceleration, presumably to overcome
the static friction force and dislodge the sled. When given a signal, they
practically leap forward, producing a substantial impact force when the chains
connecting their harnesses and the hemispheres become taut. In the photograph
taken at the site, one of the horses appears to have only one hoof on the
ground. If we make what would appear to be conservative assumptions for
acceleration, velocity, and time of interaction, we can estimate that the impact
force can easily exceed the force of atmospheric pressure.

The chambers used in Boston were 24 in. in
diameter. We did not want to make them larger for two reasons. First, to be
reasonably close to the original (20 in.), especially because of smaller number
of horses used; second, because we wanted the hemispheres to be light enough for
handling during the demonstrations. We used two 0.25- in.-thick, deep-dished
steel pieces without flanges. A commercially available bell-jar gasket was used
as the seal.

The nominal force of atmospheric pressure
for a 24-in.-diameter sealing circle is about 6550 lb. Separation is likely to
occur with a force somewhat lower than that because the gasket between
hemispheres may start leaking when most of the atmospheric force is
counter-balanced by the pulling force. We used the 0.25-in.-thick chamber edges
for sealing, without flanges, giving approximately 350 psi initial sealing
pressure on the rubber gasket. von Guericke must have had difficulties with the
seal because he used a greased leather gasket and rather thick flange with a
wide sealing surface, giving an initial sealing pressure roughly one quarter of
ours. In the model displayed in the museum, the flange surface was ~ 1 in.
wide.

One final comment: Why was the gasket
propelled into the air after separation of the chambers? (see figure). One
possibility is that the adhesion between the gasket and the edge of the mating
chamber was stronger than the adhesion to the edge to which the gasket was
attached. But, more likely, the air pressure distributions around the L-shaped
gasket at the time of separation may have produced a net outward force.

In conclusion, the lessons from the
recreation of the Magdeburg experiment are as follows:

1. Use small, tired, uncoordinated horses.
2. Feed the horses as little as possible two days before the test. 3. Build
in some cushion into the harness. For example, use nylon straps or ordinary
ropes instead of chains. 4. Tell the drivers not to synchronize the forward
signals to the two teams. 5. If money is no object, use a cylinder with a
well-sealed piston instead of hemispheres to eliminate the effect of impact
forces. 6. Finally, always use a safety chain or strap to keep things together
in case of separation.

Acknowledgments: John Sullivan (MKS)
conceived the idea of the demonstration and participated in all phase of the
project. Ted Madey (NBS) set the project into motion and obtained the necessary
funds from the AVS and its New England chapter (with the help of Chairman Tom
Shaughnessy). Larry Bell of Boston’s Museum of Science organized the
demonstration, obtained the teams of horses, provided the television tape, and
served as Master of Ceremonies. Cal Hemeon designed the details and supervised
the construction of the hemispheres.

‡This account
has been edited by T.E. Madey. The original was published in “History of
Vacuum Science and Technology”, eds. T. E. Madey and W.L. Brown (American
Institute of Physics, 1984) p. 61-63.